A conventional numerical control system for a mmachine tool or the like operates such that a pulse train is generated from a numerical control unit to drive a servo circuit, and servo motors such as pulse motors, DC motors or hydraulic motors are driven to move corresponding components or workpieces along respective axes in the machine tool or the like. However, servo and pulse motors are relatively expensive. Therefore, the overall cost of the system is increased when they are used.
However, some NC machining operations controlled by a numerical control unit do not require very high precision. For example, when a through hole is formed in a workpiece, high precision is required to index the tool with respect to the workpiece along X- and Y-axes. However, a displacement along the Z-axis need not be precisely determined since only a tip of the drill must reach through the lower surface of the workpiece so as to form the through hole.